Field of the Invention
The present invention relates to a method for estimating, in a machine tool, a temperature of a portion of which the temperature cannot be directly detected by temperature detection means such as a temperature sensor, and a method for correcting thermal displacement according to the estimated temperature.
Description of the Background Art
In a case where machining is performed by using a machine tool, thermal deformation may be caused in components of the machine tool due to, for example, mechanical heat generation by, for example, spindle and feed shaft operations, and due to temperature change of an environment in which the machine tool is installed, or temperature change of a coolant. Such thermal displacement may cause a relative position between a tool and a workpiece to be changed. Therefore, if thermal displacement occurs in the machine tool during machining, machining accuracy for the workpiece may be degraded.
In the machining, a coolant is used in many cases. In a case where a coolant is used, large displacement occurs, in some cases, due to rapid temperature change caused by difference in temperature between the coolant and the workpiece or a structure when discharging is started. Meanwhile, in a case where the discharging of the coolant is stopped, rapid temperature change occurs due to heat of vaporization generated by vaporization of water from the surface of the workpiece or the structure. Due to these influences, large displacement may occur when the machining is restarted.
As a method for inhibiting thermal displacement of a machine tool, thermal displacement correction in which temperature sensors are attached to the components of the structure of the machine tool, a displacement is calculated on the basis of the measured temperatures, and the shaft moving distance is changed according to the displacement, is widespread. However, a machine tool has many rotary portions and movable portions, and measurement of temperatures is difficult in many portions due to restriction by wiring and the like. Further, it is difficult to directly measure temperatures of a workpiece and a tool that are attached to or detached from the machine tool. Therefore, a method in which correction is performed according to a temperature of a portion of which the temperature is easily measured, instead of a portion of which the temperature is difficult to measure, may be used. However, temperatures are different at different portions. Therefore, accuracy for estimating thermal displacement is reduced, and a correction error may be increased. Further, the portion of which the temperature is difficult to measure, is a portion which is likely to be influenced by a coolant in many cases, whereby the correction error tends to be further increased.
In order to address the aforementioned problem, Japanese Patent No. 3897884 (hereinafter, referred to as Patent Literature 1) discloses a technique in which a temperature simulation block is provided in a turret head which is a rotary portion and at which attaching of a temperature sensor is difficult, and the temperature of the turret head is indirectly measured by using the temperature simulation block. The temperature simulation block is designed so as to have a heat capacity close to that of the turret head, and a coolant which has not yet been ejected is brought into thermal contact with the temperature simulation block. Patent Literature 1 indicates that, by utilizing the temperature of the temperature simulation block, thermal displacement can be accurately estimated.
Japanese Laid-Open Patent Publication No. 2002-301637 (hereinafter, referred to as Patent Literature 2) discloses a technique in which, for estimating thermal displacement of a spindle, coefficients are used so as to be different between a case where a coolant is used and a case where no coolant is used, and, when a coolant is used, the coefficient is adjusted according to a difference in temperature between the spindle and the coolant, to correct an error due to influence of the coolant.
Japanese Laid-Open Patent Publication No. 2002-326141 (hereinafter, referred to as Patent Literature 3) discloses a technique in which the tendency of temperature change of a coolant is determined, and a filter coefficient is changed between a case where the temperature rises, and a case where the temperature reduces, whereby the temperature, of a structure, influenced by a coolant is accurately estimated.
However, the method disclosed in Patent Literature 1 is performed on the assumption that machining is performed with the use of a coolant. In a case where machining is performed without using a coolant, difference in temperature between the turret head and the temperature simulation block is increased due to difference in ambient environment, and a correction error may be increased. Further, when machines are changed and the size of the turret head is changed, the temperature simulation block needs to be also changed according to the size of the turret head, and labor and time may be increased in designing and manufacturing of the machine tool.
In the method disclosed in Patent Literature 2, a coefficient is changed between a case where a coolant is used and a case where no coolant is used, to estimate thermal displacement of a spindle. Therefore, this method can be used for both a case where a coolant is used and a case where no coolant is used. However, it is difficult to use this method in practice in some points.
Patent Literature 2 indicates that the coefficient is adjusted according to difference in temperature between the spindle and a coolant, and the adjusting method is determined on the basis of an experiment. However, it may be difficult to simply adjust a coefficient based on the difference in temperature between the spindle and the coolant in practice. For example, even if difference in temperature is the same between a case where the temperature of the spindle is high, and the temperature of the coolant is close to room temperature, and a case where the temperature of the spindle is not high, and the temperature of the coolant is lower than room temperature, the influence may be different. In this case, thermal displacement needs to be measured for various temperatures of the spindle and various temperatures of the coolant, to determine the coefficients, and a large amount of experimental data is necessary. Further, Patent Literature 2 discloses a method for improving, in a transient state, change after use of a coolant is ended, but does not provide description about change after start of discharging.
In the method disclosed in Patent Literature 3, difference between a case where a coolant is used and a case where no coolant is used, is not considered, and a correction error may be increased in a case where no coolant is used.
Each of the methods disclosed in Patent Literature 2 and Patent Literature 3 cannot address rapid change that occurs in a case where difference in temperature between a workpiece or a structure, and a coolant at the start of the discharging is large.
Therefore, an object of the present invention is to provide a temperature estimation method, for a machine tool, in which a temperature of a portion of which the temperature is difficult to directly measure is accurately estimated in a simple manner, and a thermal displacement correction method which is performed on the basis of the estimated temperature.